1. Field of the Invention
The present invention relates to a developing apparatus for an electrostatic photocopier, for applying a developer to an electrostatic latent image formed on an image carrier to develop the latent image.
2. Background of the Related Art
One-component developers or two-component developers are used in a developing apparatus of this type. A two-component developer includes a toner contributing to development and a carrier for properly charging the toner. However, in such a two-component developer, a mixing ratio of the toner to the carrier, i.e., toner concentration, must be kept constant. However, it is difficult to maintain such a constant toner concentration. On the other hand, a one-component developer has an advantage in that the concentration control is not necessary since the toner for contributing to development is the only component of the developer.
One-component developers are classified into magnetic and nonmagnetic developers. Magnetic developers use magnetic materials together with nonmagnetic materials to form the developer particles. When such a magnetic developer is used in a conventional apparatus, a magnet is arranged inside a developer carrier for carrying the developer to the developing position and generating a magnetic field for supporting and carrying the developer. However, the following problems occur when the magnetic developer is used.
First, the developer carrier becomes complicated, expensive and large in size since the magnet must be supported by the developer carrier. Second a magnetic developer containing magnetic particles is more expensive than a nonmagnetic developer. Finally, since the magnetic particles do not contribute to development, color reproducibility is not satisfactory. As a result, it is difficult to perform color developing using a magnetic developer.
In order to solve the above problems, a developing apparatus using a nonmagnetic one-component developer has been used, as disclosed in the U.S. Pat. No. 4,521,098 to Hosoya et al. In Hosoya et al, a thin film layer of toner, as a nonmagnetic one-component developer which is formed on a developing roller, is pressed by only one blade, and the toner is triboelectrically charged by the blade. Thereafter, the toner is supplied to a photosensitive body on which a latent image is formed.
With the conventional developing apparatus as described above, the toner is supported on the developing roller only by triboelectrical forces therebetween. When all the toner particles are not sufficiently charged, the toner which is not contributing to development and remains on the developing roller is removed in the housing of the developing apparatus. Thus, fogging or scattering of the toner particles occurs, and a clear image may not be obtained.
To solve the above problems, a recovery blade which is urged against the developing roller has been used in the conventional developing apparatus. In this case, the residual toner on the developing roller may be recovered in the housing without fogging or scattering. However, since the recovery blade is made of an insulator, residual toner passing over the recovery blade into the housing is non-uniformly or abnormally charged by the friction therebetween. Then, the toner is non-uniformly attached to the developing roller, and forms patterns such as stripes. Therefore, a uniform thin film toner layer may not be formed on the developing roller by the above blade, so that a latent image on the image carrier may not be sufficiently developed.